1. Field of the Invention
The present invention relates to a method for recording data in an optical recording medium, an apparatus for recording data in an optical recording medium and an optical recording medium, and particularly, to a method for recording data in a write-once type optical recording medium, an apparatus for recording data in a write-once type optical recording medium and a write-once type optical recording medium which can record data by forming a recording mark having a desired length and reduce jitter of a reproduced signal.
2. Description of the Related Art
Optical recording media such as the CD, DVD and the like have been widely used as recording media for recording digital data. These optical recording media can be roughly classified into optical recording media such as the CD-ROM and the DVD-ROM that do not enable writing and rewriting of data (ROM type optical recording media), optical recording media such as the CD-R and DVD-R that enable writing but not rewriting of data (write-once type optical recording media), and optical recording media such as the CD-RW and DVD-RW that enable rewriting of data (data rewritable type optical recording media).
As well known in the art, data are generally recorded in a ROM type optical recording medium using pre-pits formed in a substrate in the manufacturing process thereof, while in a data rewritable type optical recording medium a phase change material is generally used as the material of the recording layer and data are recorded utilizing changes in an optical characteristic caused by phase change of the phase change material.
On the other hand, in a write-once type optical recording medium, an organic dye such as a cyanine system dye, phthalocyanine system dye or azo dye is generally used as the material of the recording layer and data are recorded utilizing changes in an optical characteristic caused by chemical change of the organic dye, or chemical change and physical change of the organic dye.
In this specification, a region in which an organic dye chemically changes or chemically and physically changes when a write-once type optical recording medium is irradiated with a laser beam is referred to as “a recording mark”.
When data are to be recorded in a write-once type optical recording medium, it is general that the power of a laser beam is set to a sufficiently high recording power Pw when the laser beam is to be projected onto a region where a recording mark is to be formed and that the power of the laser beam is set to a sufficiently low bottom power Pb when the laser beam is to be projected onto a region where no recording mark is to be formed, namely, a blank region, thereby projecting the laser beam onto the recording layer of the write-once type optical recording medium.
Therefore, in the region onto which the laser beam whose power is set to the recording power Pw is projected, the organic dye contained in the region of the recording layer is decomposed and transformed and the recording layer and a substrate are deformed by a pressure caused by the decomposition and transformation of the organic dye, thereby forming a recording mark. On the other hand, in the region onto which the laser beam whose power is set to the bottom power Pb is projected, neither the decomposition and transformation of the organic dye contained in the recording layer nor the deformation of the substrate occurs, so that a blank region is formed.
Thus, data can be recorded in a write-once type optical recording medium by modulating the power of the laser beam to be projected onto the optical recording medium in this manner while the optical recording medium is being rotated.
However, since heat generated in the recording layer by being irradiated with the laser beam is quickly transferred from the region onto which the laser beam is projected to ambient regions, even in regions other than the region onto which the laser beam whose power is set to the recording power Pw is projected, the organic dye contained therein sometimes is decomposed and transformed and on the other hand, in the region onto which the laser beam whose power is set to the recording power Pw is projected, deformation of the substrate sometimes fails to occur. As a result, it is sometimes impossible to form a recording mark in a desired manner.
Therefore, in the case where data are recorded in the recording layer using a simple pulse train pattern determined so that the power of the laser beam to be projected onto a region where a recording mark is to be formed is set to the recording power Pw and that the power of the laser beam to be projected onto a blank region where no recording mark is to be formed is set to the bottom power Pb, the shape of a recording mark formed in the recording layer sometimes becomes inappropriate or heat generated by the laser beam projected for forming a recording mark sometimes affects another recording mark to deform it so that a reproduced signal having good signal characteristics cannot be obtained.
These problems become prominent as the linear recording velocity becomes higher and in the case of recording data in a DVD-R in which data are normally recorded at a linear recording velocity of about 3.5 m/sec at a linear recording velocity higher than 2× speed or a linear recording velocity higher than 4× speed, these problems become particularly prominent.
Further, the same problems occur not only in a write-once type optical recording medium having a recording layer containing an organic dye but also in other kinds of a write-once type optical recording medium such as that having a recording layer consisting of a plurality of layered inorganic films.